In large-scale production processes for preparing polylactic acid (PLA), meso-lactide-containing by-products are frequently converted back into lactic acid by hydrolysis with water and are used for other purposes than for the preparation of PLA. Examples of such uses are the preparation of lactic acid esters for solvents and the preparation of alkali metal and alkaline earth metal salts of lactic acid for feedstuffs and preservatives.
For a PLA process, these uses represent a loss of yield. The uses mentioned have a lower value in most cases compared to PLA. These products are also accessible from cheaper quality lactic acid to which considerably lower requirements are applied in terms of purity than to lactic acid for the preparation of PLA. In addition, this procedure requires additional effort in terms of handling, packaging and transport.
Considered from a technical and economic standpoint, it is therefore desirable to convert also the meso-lactide-containing by-product into a technically usable PLA and thus to avoid losses.
Processes have been known which polymerize the meso-lactide to poly-DL-lactic acid (PDLLA).
Amorphous, i.e. non-crystallizable types of PLA are formed in this case which already convert into a melt at the glass transition point. This point depends somewhat on the D-lactide content of the meso-lactide used for the polymerization, which is however below 60° C. and therefore considerably lower than the melting point of semicrystalline types of PLA, which is at 135 to 180° C. The stability of amorphous types of PDLLA to hydrolysis is low, and their mechanical properties such as strength, modulus of elasticity and impact resistance are significantly lower compared to crystallizable types of poly-L-lactic acid (PLLA) having a proportion of of D-lactic acid units in the polymer chain of 0-6%. They are mostly speciality products for a very limited field of application and accordingly with low demand for quantity, for example, in medicine.
It follows that meso-lactide or a lactide mixture having relatively high proportions of meso-lactide must undergo a more costly purification than L-lactide. Only low molar mass is otherwise attained in the polymerization and yellow to brown discoloration therefore occurs in an undesired manner. Processes for polymerization of meso-lactide or meso-lactide-rich mixtures have therefore never been described as a means of improving the total yield in a large-scale production process for preparing PLA.
A poly-meso-lactide is known from WO 88/10260 (Böhringer Ingelheim) and also copolymers comprising meso-lactide units. A particularly pure meso-lactide having a melting point above 48° C. is used for the polymerization, which is prepared by rectification of a mixture of meso-lactide and D,L-lactide. In this case, D,L-lactide refers to a mixture of D- and L-lactide in a 1:1 ratio. This document contains no information on mixing this polymer with PLLA or poly-D-lactic acid nor on the properties of such a mixture.
EP 1 577 346 A1 (Mitsubishi Plastics Inc.) describes a mixture of a largely amorphous PLA and a crystalline PLA, wherein the mixing ratio is 10 to 200 parts by mass of crystalline PLA per 100 parts by mass of the amorphous PLA (corresponds to a proportion of amorphous PLA of 33.3% to 90.9%). The ratio of L-lactic acid units to D-lactic acid units in the amorphous PLA is in this case between 92:8 and 8:92 (D-proportion between 8 and 92%) and in the crystalline PLA not less than 94:6 (94% L) and not more than 6:94 (6% L).